1. Field of the Invention
This invention relates to a semiconductor laser device having a diffraction grating for producing feedback of laser light, which attains laser oscillation in a single longitudinal mode.
2. Description of the Prior Art
Semiconductor laser devices used as a light source in optical information processing systems, optical measuring systems, or other systems employing optical fibers are required to have operating characteristics that can provide laser oscillation in a single longitudinal mode. The semiconductor laser devices that can attain laser oscillation in a single longitudinal mode, that is, can emit laser light of a single wavelength, include distributed feedback (DFB) laser devices and distributed Bragg reflection (DBR) laser devices, which have a diffraction grading with a periodic corrugation formed in an active region and in the area adjacent to the active region, respectively, and emit laser light of a given wavelength.
For example, a conventional distributed feedback laser device comprises a diffraction grating with a periodic corrugation disposed on the surface of an n-type InP substrate, and an n-type InGaAs optical waveguide layer and an InGaAs active layer, both of which are disposed thereon, wherein laser light goes back and forth in the diffraction grating, resulting in laser oscillation.
In order to obtain the oscillation of laser light in the diffraction grating with a periodicity .LAMBDA., the following relation is required to hold: EQU .LAMBDA.=(N/2).multidot.(.lambda./n.sub.o) (I)
where .lambda. is the oscillation wavelength, n.sub.o is an equivalent refractive index, and N is a natural number, which denotes the order of the diffraction grating. For example, when .lambda.=1.3 to 1.5 .mu.m, n.sub.o =3.3, and N=1, .LAMBDA. is in the range of 1970 to 2350 .ANG.. That is, the periodicity of the first-order diffraction grating is in the range of 1970 to 2350 .ANG..
On the other hand, for a distributed feedback semiconductor laser device comprising a GaAlAs layer, as an active layer, formed on the GaAs substrate, which can provide an oscillation wavelength of 8900 .ANG. or less, when .lambda..gtoreq.8900 .ANG., n.sub.o =3.4, and N=1, .LAMBDA. is equal to or less than 1310 .ANG.. That is, the periodicity of the first-order diffraction grating is 1310 .ANG. or less. Moreover, as can be seen from Equation I, by increasing the order of the diffraction grating, the periodicity .LAMBDA. of the diffraction grating increases by a factor of that order.
To form such a diffraction grating, a holographic exposing system is employed that uses a He-Cd laser (wavelength .lambda..sub.o =3250 .ANG.). That is, a photoresist layer is formed on the substrate, and then exposed with an interference fringe pattern of the He-Cd laser, after which the photoresist layer thus exposed is developed to form a striped photoresist mask with a given periodicity. Using this photoresist mask, a diffraction grating with a periodic corrugation is formed on the substrate by a chemical etching technique.
The coupling efficiency of the diffraction grating increases by increasing the depth of the diffraction grating. When a diffraction grating of lower order is the same in shape and depth as a diffraction grating of higher order, the diffraction grating of the lower order has a coupling efficiency greater than that of the diffraction grating of the higher order. However, it is technically impossible to form a diffraction grating of the first order, which can be used for short wavelength AlGaAs DFB laser devices, with the conventional holographic exposing system (light source: He-Cd laser, wavelength .lambda..sub.o 3250 .ANG.).
Accordingly, a second-order diffraction grating with a periodic triangular shaped corrugation formed in the [011] direction. However, it is very difficult to form such a second-order triangular shaped diffraction grating because its periodicity .LAMBDA. is extremely small. Moreover, because the depth of the diffraction grating is small, it is difficult to form a diffraction grating with high accuracy and to obtain a high coupling efficiency.
For these reasons, the order of the diffraction grating is taken as the third order to form the diffraction grating with high accuracy, and the periodic corrugation of rectangular shape is formed in the [011] direction to obtain high coupling efficiency as compared to the periodic corrugation of triangular shape. However, even with such a rectangular shaped diffraction grating, there is a problem of how to obtain a high coupling efficiency.